In an image forming apparatus of electro-photographic system such as a laser printer, there have been heretofore employed such a process that a surface of a photosensitive member that has been uniformly charged is exposed to light by an optical beam emitted from a light emitting part that includes a laser diode or the like, thereby to form an electrostatic latent image. Then, the electrostatic latent image is formed into a visual image by toner, and thereafter, the visual image is transferred onto a recording medium such as recording paper, whereby the visual image is formed on the recording medium.
However, the image that has been simply transferred will just adheres to the recording medium by electrostatic power and dispersing power, and in that state, the image will be easily removed from the recording medium. Therefore, it is necessary to perform a fixing process for fixing the visual image on the recording medium by heating or so.
Generally, a fixing device for performing the thermal fixing process includes a heating roller and a pressurizing roller. While the recording medium such as recording paper passes between the heating roller and the pressurizing roller, developer such as toner that has been partly melted and admixed by heating will penetrate into the recording medium, and will be fixed. The heating roller contains therein a heat source such as a halogen lamp. The fixing device must have an easily exchangeable structure, because a trouble of the halogen lamp would be such a serious matter that a function as the image forming apparatus may be lost. Under the circumstances, the fixing device has been designed, as a fixing unit that integrally includes the heating roller and the pressurizing roller, and it can be easily exchanged in the event of a trouble (Refer to JP-A-8-305205, for example).